Electrolytic dissolution of stainless steel



Dec. 23, 1958 E. c. PrrzER ELECTROLYTIC DrssoLuTIoN oE STAINLESS STEELFiled June l0. 1953 lll/IIIIIIIIIIIIIIIII United 2,865,832 Patented Dec.23, 1958 ELECTROLYTIC DISSOLUTION F STAINLESS STEEL Edgar C. Pitzer,Richmond, Wash., assignor to the United States of America as representedby the United States Atomic Energy Commission Application June 10, 1953,Serial No. 360,686

Claims. (Cl. 204-222) The present invention relates to a method andapparatus for dissolving stainless steel in nitric acid.

Stainless steel usually contains chromium, nickel, iron, manganese, andother components, in varying proportions. One of the most useful andvaluable properties ot stainless steel having a chromium content inexcess of five percent is its resistance to solution in nitric acid.However, it is sometimes desirable to dissolve stainless steel in nitricacid as for example when the steel is used as a container for materialswhich must be dissolved in nitric acid.

It is accordingly an object of the present invention to provide a methodfor solubilizing stainless steel, normally resistant to solution innitric acid, in nitric acid.

It is another object of the present invention to provide an apparatusfor dissolving stainless steel in nitric acid.

It is a further object of the present invention to provide a method fordissolving stainless steel without production of large amounts ofgaseous eluents.

Other objects will be in part apparent and in part pointed out in thedescription of the invention which follows.

The present method involves subjecting the stainless steel to anodicattack in nitric acid solution. A number of problems arise when anattempt is made to electrolytically dissolve stainless steel.

One of the problems arises in connection with supplying current to thestainless steel specimen immersed in nitric acid. A conductive materialmust be employed but this material must not itself be subjected toelectrolytic dissolution while supplying current to the steel since thecurrent supplying metal itself might be dissolved bebore the solution ofthe stainless steel is complete. It has not been found practical tosupply current to the stainless steel in the electrolyte through astainless steel element which extends out of the bath because theportion of the steel specimen not immersed in the bath will not besubjected to electrolytic attack. Also the usual devices such asinsulated clamps are unsatisfactory because in dissolving specimens ofirregular shapes, portions of the specimens may be disconnected as thesupporting portions are dissolved and thus remain undissolved.

Another problem arises when it is particularly important to prevent lossof any of the components of the stainless steel or of the contents of astainless steel container. Thus, for example, when the stainless stseelis radioactive or is employed as a container for radioactive materials,it is desirable to avoid liberation of gas at the electrodes during theelectrolytic dissolution so that no radioactive contamination of thesurroundings occurs. The liberation of gases at the electrodes reducesthe efficiency of such an electrolytic process in any case becausecurrent must be consumed in effecting such liberation. For these reasonsthe use of the noble metals electrodes, such as platinum electrodes, hasnot been found satisfactory in carrying out the electrolytic dissolutionof stainless steel since platinum liberates O2 at 1.5

volts and electrolytic attack of stainless steel at lower voltages isimpractical.

These problems are overcome, according to one of the broader aspects ofthe present invention, by supplying the current through a metal whichexhibits electrolytic valve action. Such a metal is one which is notsubjected to electrolytic attack at lower voltages but which issubjected to such attack at higher voltages above the electrolytic valvevoltage. For the purposes of this application, the voltage at which theelectrolytic attack starts to take place is referred to as the valveaction voltage, or electrolytic valve voltage. When such a metal issubjected to electrolytic attack in nitric acid solution, a short surgeof current passes through the solution and a tilm is apparentlydeveloped on the surface of the metal during this short surge ofcurrent. This film is of an insulating nature since only a minutecurrent passes through the solution after the film is formed. It hasbeen discovered that, in spite of the presence of this insulating filmon valve action metals, they may satisfactorily be employed to transmitcurrent to a stainless steel element in the nitric acid electrolyte, butthat little or no gaseous eiuents are produced. It has thus been foundpossible to maintain an electrolytic attack on stainless steel articlesin nitric acid by supplying the electrolytic current to the stainlesssteel through one of the metals exhibiting electrolytic valveproperties. A maximum advantage is secured in dissolving articles ofirregular shape by forming the metal which exhibits electrolytic valveaction into a basket-like holder. A ow of electrolyzing current occursonly at the points where the stainless steel rests on the elements ofthe basket, the remainder of the basket remaining insulated due to thepresence thereon of the insulating film. As pieces of the metal specimenbecome detached and fall into contact with the basket elements,electrical contact is made through the insulating coating at the pointwhere they contact the basket to result in their electrolyticdissolution. The ow of electrolyzing current from the basket surface isthus selectively restricted to only those points where the basket iscontacted by the stainless steel.

The accompanying drawing is a diagrammatic illustration, partly insection, of an electrolytic cell and the circuit diagram of an apparatussuitable for carrying out the present invention.

Although it is possible to dissolve most of the components of stainlesssteel, a number of these components, including columbium, columbiumcarbide and silicon, are not brought into solution. The release of thesecomponents, as the soluble metals are dissolved from the stainlesssteel, results in the deposit of an insoluble sludge between thestainless steel being subjected to electrolytic attack and thesupporting metal. The deposit of this layer of sludge raises theresistance between the holder metal and stainless steel specimen andrequires the voltage impressed on the holder to be raised in order todeliver the same amount of current to the stainless steel specimen. Ifthe voltage which must be impressed to continue the electrolyticdissolution of the stainless steel exceeds the electrolytic valve actionvoltage, the anode metal itself is subjected to electrolytic attack bythe nitric acid. This difficulty is overcome according to the presentinvention by providing a relative movement of the holder metal andstainless steel in order to dislodge the sludge and maintainelectrolytic contact therebetween.

The advantages of the present invention may be best understood from aconsideration of an illustrative embodiment of the invention whichfollows. It will be understood that the scope of the invention is notconned to this illustrative embodiment.

Referring to the figure, a solution of nitric acid electrolyte 10 isdisposed in a container 12 composed of an material.

insulating material. A metal basket 14 composed of a metal exhibitingelectrolytic valve action is supported in the solution from a hook 16composed of an insulating This hook extends from ya vibrator 18, theoperation of which will be described below. A cathodic electrode 20 isalso included in the electrolytic bath.

`A source of direct current 22 is used to carry out the electrolyticoperation. This current is supplied to ,the .basket 14 through theelectrical leads 21 and 24 and the conducting handle 26 of the basket14. A negative potential is established on the cathode 20 by connectingthe negative pole of the current source 22 with cathode 20 through theelectricallead 28, rheo-stat 30, the por tion 29 of the potentiometer27-29, the conductor 32, rheostat 34 and conductor 36. A voltmeter isconnected between electrical leads 24 and32 and an ammeter is connectedin the electrical lead 24.

The vibrator 18 imparts a vibrating motion to the insulated hook support16. Current is supplied to the vibrator 1 8 from alternating currentsource 46 by various conductors depending on its mode of operation. Theoperation of thevibrator may be made continuous so that the basket 14 ismade to vibrate during the entire electrolytic operation or it may bemade intermittent so that it is brought into operation only when thepotential between the electrodes rises above a certain value. Thepositioning of a single pole, double throw switch 54 determines whetherthe operation is continuous or intermittent.

For continuous vibration, the switch 54 is connected to terminal 51, asshown in the figure, so that a direct connection is made between thecurrent source 46 and the vibrator 13, through conductors 49, 50 and 52and through switch 54.

When the vibrator is to be operated intermittently, the switch 54 sconnected to thepole 53 so that a relay 40 is included in the vibratorcircuit. When the switch is so connected, current is supplied to thevibrator only when the relay 40 is activated. When so activated,vcurrent ows from current source 46 through conductors 49, 4S, 47, 50 and52 and through switch 54 and relay 40 to operate vibrator 18. Relay 40may be any conventional relay whose winding is energized at apredetermined voltage and acts to connect conductor 47 to conductor 48.The relay winding is connected across the positive terminal 31 andnegative terminal 33 by means of conductors 42 and 44. Also connectedbetween terminals 31 and 33 is the portion 27 of potentiometer 27-29. Itcan be seen that the resistance of the electrolyte is in series with theresistance of therheostat 34 and this series combination is similarlyconnected across terminals 31 and 33.

Since resistance 27 of potentiometer 27-29 and the series combination10and 34 are in parallel, the total resistance between terminals 23 and25 of potentiometer 27-,29 may be changed either by changing the posi`tion ofthe moving terminal 25 orby changingthe resislaneof vtheseriescombination 10 and 34. [Itis apparent that a change ,in this totalresistance will result in a proportional change in the voltage appearingbetween terminals 23 and 25.

.During the course of the electrolytic dissolution, a deposit of asludge of the insoluble components of stainless steel forms between thesurface of the basket 14 and the undissolved stainless steel. Thiscauses an increase inthe resistance of cell 10 and acorresponding risein the voltage appearing between the terminals 23 and 25 of thepotentiometer 27- -29.y This voltage riseis applied across the windingof relay 40 and, when the voltage reaches a predetermined value, therelay winding is energized and in turn activates the vibrator 18 byconnecting conductors 47 and 48. Activation of the vibrator removes thesludge and reduces the cell resistance. This in turn reduces: the-voltage kappearing between terminals 23 and 25 untiLtherelaywindingisde-energizediand the v ibra- Example l A basket with a square crosssection of 11/2 inches and a depth of 5 inches was Welded from titaniumsheet and perforated with 1&2 inch holes about 1A inch apart. The basketwas filled with 100 grams of stainless steel pin tubing in 7-inchlengths and immersed in three liters of 4 molar nitric acid. Thestainless steel had the following composition-about 71% iron, about 18%chromium, about 8% nickel, about 2% manganese, about 1% columbium andfractional percentages of silicon and carbon. A stainless steel strip,of the same composition was used` as the cathode. An electrolyzingcurrent of 5 to 6 amperes was applied at a voltage of 3.0 to 3.1 voltsemploying the circuit illustrated in the figure. The relay 40 wasadjusted so that the basket was vibrated whenever the cell voltage roseto about 8 volts and stopped Operating when the voltage fell below 6volts. After .about three hours, the voltage was raised to 6 to 9.5volts and the current to 20 amperes with a corresponding adjustment inthe relay to accommodate this change in voltage. After a total of l0hours operation (3 hours atthe lower voltage and 7 hours at the higher),the cell was dismantled. Only a very small residue of stainlesssteelslivers remained undissolved. Approximately 141 ampere hours .hadpassed during the totaloperation so that the current yield was about0.71 grams per ampere hour. The loss of weight of the titanium basketwas only 0.93 gram out of a total initial weight of 109.55 grams.

Example Il A cylindrical basket, 21/2 inches in diameter and 30 inchesin length, was formed from tantalum sheet of 30 mil thickness. It wasperfo-rated with 1/32 inch holes at approximately 1A inch spacing.. Thebasket was loaded with about 500 grams of stainless steel of the samecomposition given in Example I in the form of stainless steel pintubing. The basket and a stainless steel strip cathode, 31/2 inches Wideand 24 inches long, were immersed in 14 liters of 4 molar nitric acid.Electrolysis was begun at about 6 volts and 90 to 100 amperes. Afterabout 6 hours, the voltage was reduced to 4 volts and the current to 70amperes in order to avoid boiling of the solution. The vibrator wasoperated during the entire run so as to shake the basket in a verticalplane over a wide range of frequencies and over a range of amplitudesfrom 0 to 1A inch. The charge of pin tubing was completely dissolved inthe 16 hours. Evolution of gas at the cathode was almost negligibleduring most ofthe electrolysis.

Example lll Approximately 492 grams of stainless steel tubes,30 inchesin length and containing 546 grams of a 63/27 mixture of UOzzMgOZ, wereplaced inthe same basket described in Example Il. The basket, togetherwith the corresponding strip cathode, was immersed in 17 liters of 4molar nitric acid and the basket was shaken as described in Example II.The electrolytic bath was surrounded by crushed ice so that thetemperature of the electrolyte remained between 0 and'20 C. for most ofthe run. The initial voltage applied was 7 volts at 90 amperes. Thecurrent remained fairly constant'for"7 hours but liuctuated ,after thisuntil the dissolution of the assembly was completed at the end of 8%hours. The solutionwas then heated to about C. for 4 hours in `order tocomplete the dissolution of U02. After this solution .was-.centrifugedin 'order `to .recover and wash the residual sludge of insolublecomponents. Upon analysis, the sludge was found to contain only 0.05% ofthe uranium originally present in the assembly. Approximately 709.3ampere hours passed during the electrolytic dissolution corresponding toan anodic dissolution yield of 0.69 grams of stainless steel per amperehour. Gas evolution from the cathode was again relatively small inamount.

From the foregoing, it is apparent that the present invention provides anovel and useful method for dissolving stainless steel, normallyinsoluble in nitric acid, in nitric acid. It is further apparent thatthe electrolytic dissolution can be carried out according to the presentinvention using relatively low voltages below the electrolytic valvevoltage of the metal basket used to transmit current to the stainlesssteel.

Metals suitable for this purpose must of course be insoluble in nitricacid and must exhibit an electrolytic valve action. Among such metalsare columbium, tantalum, titanium and zirconium. Tantalum remainspassive up to 120 volts in 4 molar nitric acid, titanium is similarlypassive up to l2 volts and zirconium up to 9() volts.

To dissolve stainless steel electrolytically, a minimum voltage of 1.3volts must be applied and the maximum voltage is that just below thevalve action voltage for each valve action metal.

Although 4 molar nitric acid has been found most economical in carryingout the present process, either more or less concentrated acid may beused.

Since many embodiments might be made of the present invention and sincemany changes might be made in the embodiment described, it is to beunderstood that the foregoing description is to be interpreted asillustrative only and not in a limiting sense.

I claim:

1. Apparatus for substantially completely dissolving radioactivestainless steel elements in nitric acid, the steel of said elementshaving a high chromium content, said apparatus comprising anelectrolytic cell adapted to contain nitric acid, a source ofelectrolyzing current, means for supplying said electrolyzing current tosaid elements in said bath, said means comprising a cathode and an anodeholder in said cell, said anode holder being composed entirely of ametal selected from the group consisting of tantalum, columbium,titanium and zirconium, said anode holder being adapted to receive andsupport said stainless steel elements and being coated with a lm formedby subjecting said metal to electrolytic attack in nitric acid wherebysaid holder is adapted to supply electrolyzing current to said steelelements in nitric acid bath only at points where said elements rest incontact with said anode holder and means for agitating said holder.

2. The apparatus of claim 1 in which the selected metal is tantalum. Y YY Y 3. The apparatus of claim 1, in which the selected metal istitanium.

4. The apparatus of claim 1 in which the selected metal is zirconium.

5. The apparatus of claim 1 in which the selected metal is columbium.

References Cited in the le of this patent UNITED STATES PATENTS Re.23,068 Feild Jan. 4, 1949 578,953 Thum Mar. 16, 1897 1,682,426 SchutteAug. 28, 1928 1,970,804 Kerk Aug. 2l, 1934 2,058,365 Stark Oct. 20, 19362,564,823 Wallace Aug. 21, 1951 2,695,872 Espy NOV. 30, 1954 2,708,181Holmes et al. May 10, 1955 2,782,160 Treuhaft et al. Feb. 19, 1957FOREIGN PATENTS 4,624 Great Britain Feb. 24, 1898

1. APPARATUS FOR SUBSTANTIALLY COMPLETELY DISSOLVED RADIOACTIVE STAINLESS STEEL ELEMENTS IN NITRIC ACID, THE STEEL OF SAID ELEMENTS HAVING A HIGH CHROMIUM CONTENT, SAID APPARATUS COMPRISING AN ELECTROLYTIC CELL ADAPTED TO CONTAUN NITRIC ACID, A SOURCE OF ELECTROLYZING CURRENT, MEANS FOR SUPPLYING SAID ELECTROLYZING CURRENT TO SAID ELEMENTS IN SAID BATH, SAID SAID MEANS COMPRISING A CATHODE AND AN ANODE HOLDER IN SAID CELL, SAID ANODE HOLDER BEING COMPOSED ENTIRELY OF A METAL SELECTED FROM THE GROUP CONSISTING OF TITALUM, COLUBIUM, TITANIUN AND ZICONIUM, SAID ANODE HOLDER BEING ADAPTED TO RECEIVE AND SUPPORT SAID STAINLESS STEEL ELEMENTS AND BEING COATED WITH A FILM FORMED BY SUBJECTING SAID METAL TO ELECTROLYTIC ATTACK IN NITRIC ACID WHEREBY SAID HOLDER IS ADAPTED TO SUPPLY ELECTROLYZING CURRENT TO SAID STEEL ELEMENTS IN NITRIC ACID BATH ONLY AT POINTS WHERE SAID ELEMENTS REST IN CONTACT WITH SAID ANODE HOLDER AND MEANS FOR AGITATING SAID HOLDER. 